Kahalalide F is a bioactive cyclic depsipeptide isolated from the sarcoglossan mollusc Elysia rufescens and its diet, the green alga Bryopsis sp. Kahalalide F was first isolated by Hamann and Scheuer, see Hamann, M. T.; Scheuer, P. J. J. Am. Chem. Soc. 1993, 115, 5825-5826. In this publication, the absolute stereochemistry of individual valines (3 D-Val and 2 L-Val) and of threonines (L-Thr and D-allo-Thr) was not determined. In a later publication, Scheuer et al. (Goetz, G.; Yoshida, W. Y.; Scheuer, P. J. Tetrahedron 1999, 55, 7739-7746) assigned a position in the molecule for the 5 Val and the 2 Thr.
Thus, the structure for Kahalalide F according to Scheuer et al. was:

In the meantime, Prof. Reinhart also assigned the individual position of the 5-Val and the 2-Thr (K. L. Rinehart, personal communication) While for the 2 Thr and 3 Val, his assignation concords with that of Prof. Scheuer, for the last 2 Val there is a discrepancy. Thus, in the Rinehart assignment, the two consecutive Val in the side chain are switched and the structure is of the formula (I):

It is now accepted, and demonstrated again in this text, that the correct formula for kahalalide F is the formula (I).
The structure is complex, comprising six amino acids as a cyclic part, and an exocyclic chain of seven amino acids with a terminal acyl group. Kahalalide F (I) is an exceedingly potent and rare marine-derived antitumour agent, though the absence of adequate quantities has slowed plans for clinical trials.
Other kahalalide compounds are known, and for example we refer to Hamann, M. T., et al., J. Org. Chem., 1996, “Kahalalides: Bioactive Peptides from Marine Mollusk Elysia rufescens and its Algal Diet Bryopsis sp.”, vol. 61, pp. 6594-6660. As well as kahalalide F, this article gives structures for kahalalides A to E. Kahalalide K was reported in J Nat. Prod. 1999, 62, 1169, and kahalalide O was reported in J Nat Prod 2000, 63, 152.
The structures for these further kahalalide compounds are shown in the following formulae:

The cyclic kahalalides have a ring of amino acids, and a side chain which terminates in an acyl group.
Kahalalides have a range of biological activities, notably antitumour and antituberculosis activity.